The present invention describes a membrane-electrode assembly (“MEA”) for use in PEM water electrolysers. In addition, the membrane-electrode assembly can also be used for regenerative fuel cells (RFCs) or for oxygen-producing electrodes in various other applications of electrolysis. Furthermore, a process for producing the membrane-electrode assembly is described.
In a future energy economy based on renewable resources, hydrogen will become an important energy carrier. The electrolysis of water is the most practicable method of producing hydrogen using renewable energy sources. The capital and production costs for electrolysers determine the overall economics of the system and will therefore determine whether this becomes a practical process for producing hydrogen. The costs of production of hydrogen by electrolysis of water are influenced to a large extent by the consumption of electric energy, which can make up about 70% of the total costs for the production of hydrogen.
According to the present state of the art, use is usually made of two different types of cell for the electrolysis of water, namely alkaline electrolysers and electrolysers which are provided with a polymer electrolyte membrane (“PEM”). Water electrolysers which utilize a PEM in combination with noble metal catalysts are able to operate at significantly higher current densities and thus with a lower specific energy consumption compared to conventional, alkali-containing electrolysers, so that they have the advantage of higher output of the plants and lower production costs. The present invention therefore has the object of improving the process of electrolysis of water by means of PEM electrolysers and in particular of providing improved membrane-electrode assemblies (MEAS) for PEM electrolysers.
PEM electrolysers generally have a similar structure to a PEM fuel cell, but they operate in a different way. During operation of the PEM fuel cell, reduction of oxygen takes place at the cathode and oxidation of hydrogen takes place at the anode of the fuel cell. The end effect is that water and electric power are produced. On the other hand, flow of current and electrodes are reversed in a PEM electrolyser, so that decomposition of water takes place.
The liberation of oxygen occurs at the anode (“oxygen evolution reaction” or “OER” for short) and the reduction of protons (H+), which pass through the polymer electrolyte membrane, takes place at the cathode (“hydrogen evolution reaction” or “HER” for short). The result is that water is decomposed into hydrogen and oxygen with the aid of electric current. The reactions can be summarized by the following equations:2H2O=>O2+4H++4e− (OER)4H++e−=>2H2 (HER)
An MEA for a PEM water electrolyser (hereinafter also referred to as “electrolysis MEA”) generally contains a polymer electrolyte membrane (for example Nafion® from DuPont) which is arranged in the manner of a sandwich construction between two electrodes and two porous current collectors (or gas diffusion layers) which are each mounted on the two sides of the electrodes.
However, owing to the different requirements which electrolysis MEAs have to meet and the different operating conditions of electrolysers and conventional PEM fuel cells, there are important differences in the requirement profile for electrolysis MEAs:    (a) Owing to the corrosion which can be caused by the oxygen formed on the anode side in the OER, materials based on carbon (for example Pt/C catalysts supported on carbon black or gas diffusion layers “GDLs” based on carbon fibres) cannot be used on the anode side of an electrolysis MEA.    (b) The electrolysis process is frequently carried out under elevated pressure on the hydrogen side in order to carry out a precompression for the storage of the hydrogen. At present, pressures of up to 15 bar, in exceptional cases up to 30 bar, are reached. This means that the electrolysis MEA is subjected to a differential pressure between anode and cathode which is from about 5 to 10 times as high as in the operation of a conventional PEM fuel cell. This places increased demands on the stability and pressure resistance of the MEA. Preference is therefore given to using relatively thick membrane materials (up to a thickness of 200 μm). However, new MEA construction concepts as described in the present patent application are also necessary to increase the pressure stability.    (c) Since not only hydrogen but also oxygen is liberated during the electrolysis process, there is a latent risk of a hydrogen/oxygen gas explosion in the case of leakage. The reactants have to be strictly separated from one another to avoid such effects. This places increased demands on the gastightness of the electrolysis MEAS.    (d) Furthermore, different catalysts have to be used for electrolysis MEAs. Iridium is known for its unique electrocatalytic properties in respect of processes for the generation of chlorine and oxygen. Iridium is therefore the preferred material for the oxygen evolution reaction (OER) on the anode side, either in the form of the pure metal (as “black”) or as oxide, if appropriate in admixture with other oxides. Suitable anode catalysts for electrolysis MEAs are described, for example, in the German Patent Application P 1 0350 563.6 of the applicant. Among all precious metals, platinum is the most active catalyst for the hydrogen evolution reaction (HER) at the cathode and is frequently used as cathode catalyst in electrolysis MEAs.
For these reasons, conventional MEAs as are used for PEM fuel cells cannot be used for PEM electrolysers.
Various proposals for the construction of electrolysis MEAs have become known from the patent literature. US 2003/0057088 A1 describes a PEM water electrolyser comprising MEAs which comprise an ionomer membrane, two catalyst layers and a pair of porous current collectors and are pressed in a sandwich-like manner between two electrode plates. The catalyst layers are applied on the front and rear sides of the membrane by the “decal” process. The catalyst layers, the gas diffusion layers and the membrane have the same dimensions (“coextensive design”), and the use of seals is not described.
WO 02/27845 A2 discloses a water electrolysis cell which has an “integral membrane support and frame structure” for the ionomer membrane. The catalyst layers are applied on both sides of the membrane, with large proportions of the membrane not being coated in the peripheral region. This results in a considerably increased consumption of expensive ionomer membrane, which leads to higher costs of the PEM electrolyser.
U.S. Pat. No. 6,613,215 B2 describes a PEM electrolyser containing an ultrathin composite membrane. Anode and cathode catalysts are applied on the front and rear side, respectively, of the membrane, once again with large proportions of the membrane not being coated and additional costs being incurred as a result.
The processes for producing electrolysis MEAs are in principle similar to the processes for producing conventional membrane-electrode assemblies (MEAs) for PEM fuel cells. In general, catalyst inks comprising catalyst powder, solvent and optionally polymer electrolyte material (i.e. a dissolved ionomer) are prepared and either applied directly to the ionomer membrane or firstly applied to the gas diffusion layer and then joined to the membrane (cf., for example, the patents U.S. Pat. Nos. 5,861,222; 6,309,772 and 6,500,217 of the applicant). Problems with accurate positioning and dimensional stability of the motifs occur particularly in the two-sided coating of the ionomer membranes.
It was therefore an object of the present invention to provide an electrolysis MEA which, owing to its structure, has improved pressure stability at high differential pressures (up to 30 bar) and also has improved gastightness. The electrolysis MEA should be able to be produced in a simple, inexpensive process without a high membrane consumption. The process should have low failure ranges and a high accuracy of fit and thus be suitable for mass production.
This object and others are achieved by the membrane-electrode assemblies (MEAs) of the present invention. In addition, the present invention provides processes for producing the MEAs and products for their use.